Struggling with SQL subquery selection - sql

Question

I'm trying to answer a SQL question for revision purposes but can't seem to work out how to get it to work. The tables in question are:
The question is asking me to write an SQL command to display for each employee who has a total distance from all journeys of more than 100, the employee's name and the total number of litres used by the employee on all journeys (the number of litres for a journey is distanceInKm / kmPerLitre).
So far I've tried several variations of code beginning with:
SELECT
name, TravelCost.distanceInKm / Car.kmPerLitre AS "Cost in Litres"
FROM
Employee, Car, TravelCost
WHERE
Employee.id = TravelCost.employeeID
AND Car.regNo = TravelCost.carRegNo
It's at this point I get a bit stuck, any help would be greatly appreciated, thanks!

Answer 1

Never use commas in the FROM clause. Always use proper, standard, explicit JOIN syntax.
You are missing a GROUP BY and a HAVING:
SELECT e.name, SUM(tc.distanceInKm / c.kmPerLitre) AS "Cost in Litres"
FROM Employee e JOIN
TravelCost tc
ON e.id = tc.employeeID JOIN
Car c
ON c.regNo = tc.carRegNo
GROUP BY e.name
HAVING SUM(tc.distanceInKm) > 100;

Answer 2

Use Group By and Having Clause
SELECT NAME,
Sum(TravelCost.distanceInKm/ Car.kmPerLitre) AS "Cost in Litres"
FROM Employee
INNER JOIN TravelCost
ON Employee.id = TravelCost.employeeID
INNER JOIN Car
ON Car.regNo = TravelCost.carRegNo
GROUP BY NAME
HAVING Sum(distanceInKm) > 100

Answer 3

You need to JOIN all the tables and find sum of litres like this:
select
e.*,
sum(distanceInKm/c.kmPerLitre) litres
from employee e
inner join travelcost t
on e.id = t.employeeId
inner join car c
on t.carRegNo = c.regNo
group by e.id, e.name
having sum(t.distanceInKm) > 100;
Also, you need to group by id instead of just names as the other answers suggest. There can be multiple employees with same name.
Also, use explicit JOIN syntax instead of older comma based syntax. It's modern and clearer.

Answer 4

-- **How fool am I! How arrogant am I! I just thought `sum(tc.distanceInKm/c.kmPerLitre)`
-- may have a problem, since a employee may have multiple cars,and car's kmPerLitre is differenct.
-- However there is no problem, it's simple and right!
-- The following is what I wrote, what a bloated statement it is! **
-- calcute the total number of litres used by the employee on all journeys
select e.name, sum(Cost_in_Litres) as "Cost in Litres"
from (
select t.employeeID
-- calcute the litres used by the employee on all journeys group by carRegNo
, sum(t.distanceInKm)/avg(c.kmPerLitre) as Cost_in_Litres
from TravelCost t
inner join Car c
on c.regNo = t.carRegNo
where t.employeeID in
( -- find the employees who has a total distance from all journeys of more than 100
select employeeID
from TravelCost
group by employeeID
having sum(distanceInKm)> 100
)
group by t.carRegNo, t.employeeID
) a
inner join Employee e
on e.id = a.employeeID
group by e.id,e.name;